1. Field of the Invention
The present invention relates to a spark-timing control system for use with an internal combustion engine having means for cutting off the supply of fuel to the engine under specified engine operating conditions, including engine deceleration.
2. Description of the Prior Art
The amount of fuel to automotive vehicle internal combustion engines is controlled by fuel control devices such as fuel injection devices or carburetors. Fuel cut-off systems have already been proposed for use with such fuel control devices to cut off the supply of fuel to the internal combustion engines during the deceleration mode of engine operation. This is effective to achieve high fuel economy and high exhaust emission purifying efficiency and to prevent exhaust emission purifying catalytic converters from overheating during engine deceleration.
It is a common practice to cut off the supply of fuel to an internal combustion engine in accordance with several engine operating conditions such as throttle position, engine speed and engine coolant temperature, for example, when the throttle valve is in its idle position, the engine speed is above a predetermined value, and the engine coolant temperature is above a predetermined value.
One difficulty with such conventional systems is that an engine torque change occurs causing an uncomfortable shock, when the engine operation is shifted between its fuel cut-off and fuel-delivery modes. Particularly upon fuel delivery resumption, the engine torque changes rapidly from zero value to produce an uncomfortable shock.
The present invention provides a spark-timing control system which can retard the spark-timing from the optimum spark-timing when the engine accelerates under a fuel cut-off condition. This operation is effective to achieve a smooth engine output torque change and to prevent the shock previously attendant upon engine re-acceleration.